Included within the processing steps for the fabrication of a semiconductor package, a semiconductor die or chip is mechanically bonded with an adhesive to a substrate. The fabrication can take place in a continuous series of steps, or the substrate can be prepared with the adhesive for the mechanical attach, and then held until a later time.
If the fabrication process is to be interrupted after the deposition of the adhesive to the substrate and the assembly held to a later time, the adhesive must be in a solidified form for successful storage. The solidified form provides the further advantages of minimal or no bleeding, and better control of bondline thickness and bondline tilt, the bondline being the interface between the chip and the adhesive.
For some semiconductor packaging applications, paste adhesives are preferred over solid (film) adhesives for process reasons, yet the bond-line and fillet control of solids are desired. In such a case, an adhesive known as a B-stageable adhesive may be used. If the starting adhesive material is a solid, the solid is dispersed or dissolved in a solvent to form a paste and the paste applied to the substrate. The adhesive is then heated to evaporate the solvent, leaving a solid, but uncured, adhesive on the substrate. If the starting adhesive material is a liquid or paste, the adhesive is dispensed onto the substrate and heated to partially cure the adhesive to a solid state. The application of heat at this stage in fabrication is termed B-staging, and the adhesive, B-stageable.
Although there are the advantages to solid adhesives mentioned above, there are also disadvantages. After B-staging and during storage, solid adhesives are prone to absorbing moisture from the air under ambient conditions, or from substrates, especially organic substrates such as BT resins, printed circuit boards or polyimide flexible substrates. The adhesives also may contain a level of residual solvent or other volatiles.
At elevated attach temperatures, the absorbed moisture and residual volatile materials will evaporate rapidly. If this evaporation occurs faster than the vapors can diffuse out of the adhesive, voids or bubbles appear in the adhesive and can be a source of ultimate failure of the adhesive. This creates a need for curable compositions that are B-stageable but that do not promote voiding.
Cyclic or linear siloxane resins, such as those disclosed in U.S. Pat. Nos. 4,751,337, 4,877,820, 4,900,779, 4,902,731, 5,013,809, 5,025,048, 5,077,134, 5,118,735, 5,124,423, 5,124,375, 5,147,945, 5,171,817, 5,196,498, 5,242,979, 5,260,377, 5,334,688, 5,340,644, 5,373,077, 5,391,678, 5,408,026, 5,412,055, 5,451,637, 5,491,249, 5,523,374, 5,512,376, have pendant carbon to carbon double bonds introduced through the addition of dicyclopentadiene to the linear or cyclic siloxane backbone. These siloxane compounds have superior stability and very low moisture absorption, however, they have less than optimum adhesive properties.
Oxetane resins are highly reactive cyclic ethers that can undergo both cationic and anionic ring opening homopolymerization, and in general, exhibit good adhesion, shrink minimally upon cure, and polymerize readily. A combination of the properties found in oxetane compounds and in siloxane compounds, in which the siloxane compounds contain Si—H bonds and carbon to carbon double bond functionality, would be an advantage in uses requiring dual cure materials, such as in B-stageable adhesives for use in electrical, electronic, photonic, and optoelectronic applications.